Ceramic filters are used in radio equipment to reduce interference. In particular, ceramic duplex filters including two individual band-pass filters are often employed. For example, one filter connects the receiving branch and has a center frequency and bandwidth corresponding to the receiving band, while the other filter connects the transmission branch and has a center frequency and bandwidth corresponding to the transmission band.
Existing recessed top pattern (RTP) filters are large, which limits their reliable manufacture. Furthermore, such filters are architecturally limited by the requirement that the strongest bulk field couplings only occur to the next in-line resonator (without skip-resonator field couplings that would improve filter roll off). Also, such filters have undesired inter-resonator couplings due to the in-line architecture and the unconstrained fields around each resonator.
Conventional ceramic monoblock filters have resonator-to-resonator couplings limited to a straight-line path. This is due to each resonator having a strong coupling tendency primarily to its nearest neighbors within the ceramic block. This restriction to a straight-line path limits the design options for the filter. In turn, this limits the achievable filter response and the achievable small size of the filter based on the required filter response. There also exists strong undesired coupling between resonators in the straight-line path that are not adjacent. This is due to unimpeded electromagnetic fields that extend into the entire monoblock structure.
Thus, there is a need to couple resonators that are not in a straight-line path. There is a further need to permit coupling between resonators in adjacent blocks in a duplex filter, and also to prevent coupling between some resonators in the straight-line path of each block.